The term chalcedony, derived from χαλκηδών the name of a town in Asia Minor, is usually confined to stones of a greyish tinge. Stones artificially coloured an emerald green have been cut and put upon the market as ‘emeraldine.’ Carnelian is a clear red chalcedony, and sard is somewhat similar, but brownish in tint. Chrysoprase is apple-green in colour, nickel oxide being supposed to be the agent. Prase (cf. p. 240), which is a dull leek-green in hue, may also in part be referred here; the name comes from πράσμον, a leek. Plasma, which may have the same derivation, is a brighter leek-green. Jasper is a chalcedony coloured blood-red by iron oxide, while bloodstone is a green chalcedony spotted with jasper; they are popular stones for signet rings. Flint, an opaque chalcedony, breaks with a sharp cutting edge, and was much in request with early man as a tool or a weapon; its property of giving sparks when struck with steel rendered it invaluable before the invention of matches. Hornstone is somewhat similar, but more brittle, while chert is a flinty rock. Agate, named after the river Achates in Sicily, where it was found at the time of Theophrastus, has a peculiar banded structure, the bands being usually irregular in shape, following the configuration of the cavity in which it was formed. Moss-agate, or mocha-stone, contains moss-like inclusions of some fibrous mineral. Onyx is an agate with regular bands, the layers having sharply different colours; when black and white, it has, in days gone by, been employed for cameos. Sardonyx is similar in structure, but red and white in colour. Agate is used in delicate balances for supporting the steel knife-edges of the balance itself and of the panholders, and is largely employed—especially when artificially coloured—for umbrella handles and similar articles. Chalcedony and agate are found the whole world over, but India, and particularly Brazil, are noted for their fine carnelians and agates. CHAPTER XXX OPAL (_White Opal_, _Black Opal_, _Fire-Opal_)